1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling a soft buffer for carrier aggregation in a wireless communication system and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3GPP Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3rd Generation Partnership Project (3GPP) as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved UTRAN (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple user equipments (UEs), and communicates with a core network including a mobility management entity (MME), a serving gateway, etc., for Non Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at coverage edge of an eNB. Besides, the LTE-A system includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint transmission/reception (COMP), UL multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
The CA is introduced to the LTE-A system by which more than one component carriers (CCs) are aggregated to achieve a wider-band transmission. Accordingly, the LTE-A system can support a wider bandwidth up to 100 MHz by aggregating a maximum number of 5 CCs, where a maximum bandwidth of each CC is 20 MHz and is backward compatible with the 3GPP Rel-8 standard. The LTE-A system supports the CA for both contiguous and non-contiguous CCs, wherein each CC limited to a maximum of 110 resource blocks. The CA increases bandwidth flexibility by aggregating the CCs.
When a UE is configured with the CA, the UE has the ability to receive and/or transmit packets on one or multiple CCs to increase throughput. In the LTE-A system, it is possible that an eNB configures the UE different numbers of uplink (UL) and downlink (DL) CCs. Moreover, the CCs configured to the UE necessarily consists of one DL primary CC (PCC) and one UL PCC. The most important feature of the DL PCC and the UL PCC is exchanging control information between the UE and the eNB. CCs other than the PCCs are named UL or DL secondary CCs (SCCs). Numbers of the UL and DL SCCs are arbitrary, and are related to UL and DL aggregation capabilities of the UE and available radio resources.
A hybrid automatic repeat request (HARQ) process is used in the LTE system to provide both efficient and reliable communications. Different from an ARQ process, a forward correcting code (FEC) is used for the HARQ process. For example, a receiver feeds back an acknowledgment (ACK) to inform a transmitter that a packet has been received correctly if the receiver decodes the packet correctly. Oppositely, the receiver feeds back a negative acknowledgment (NACK) to the transmitter if the receiver cannot decode the packet correctly. In this situation, the UE stores part or the whole of the packet in a soft buffer of the UE. After the UE receives a retransmitted packet from the transmitter, soft values of the retransmitted packet and the stored packet are combined. The receiver decodes the packet by using the combined soft values. Furthermore, the combination of the previously erroneously received packet (s) and the currently received packet increases a probability of successful decoding. The UE continues the HARQ process until the packet is decoded correctly, or until a maximum number of retransmissions have been sent, at which time the HARQ process declares a failure and leaves it up to the ARQ process in radio link control (RLC) for trying again. In other words, space of the soft buffer should be reserved for the HARQ process such that the UE can store the HARQ process which has not been decoded correctly. Otherwise, the UE blocks the HARQ process if the soft buffer is fully occupied. When multiple packets are transmitted to the UE, the UE may need to store multiple HARQ processes due to unsuccessful decoding of the packets.
In detail, the UE can store up to 8 HARQ processes in the soft buffer in the LTE system (i.e., single CC system). A transport block (TB) is a physical interface between an eNB and a UE, and corresponds to the data carried in a LTE radio subframe. Further, each LTE radio subframe is 1 millisecond (ms), and each LTE radio frame is 10 ms, which consists of 10 LTE radio subframes. When using the MIMO (e.g. spatial multiplexing), more than one transport blocks can be transmitted per transmission time interval (TTI) for the UE.
A soft buffer partition rule in the LTE system (i.e., single CC system) is introduced as follows. The total number of soft channel bits, Nsoft, depends on UE category of the UE, as shown in a table 10 of FIG. 1, wherein various values of Nsoft are listed according to an example of the prior art. Nsoft can be divided into multiple partitions according to the following equation:
                                          N            IR                    =                      ⌊                                          N                soft                                                              K                  MIMO                                ·                                  min                  ⁡                                      (                                                                  M                        DL_HARQ                                            ,                                              M                        limit                                                              )                                                                        ⌋                          ,                            (                  Eq          .                                          ⁢          1                )            wherein NIR is a size of a partition which is used for storing a transport block. Nsoft is the total number of soft channel bits of the UE. KMIMO is a number of transport blocks that may be transmitted to the UE in a TTI, and is related to the MIMO used by the UE and the network. In general, if the spatial multiplexing with n spatial streams is configured to the UE, KMIMO is set to n. Mlimit is a positive value which equals to 8. MDL—HARQ is a maximum number of DL HARQ processes per serving cell, and corresponds to a duplex mode and its configuration. For example, MDL—HARQ is set to 8 for frequency-division duplexing (FDD). Values 4, 7, 10, 9, 12, 15 and 6 are used for time-division duplexing (TDD) UL/DL configuration 0, 1, 2, 3, 4, 5 and 6, respectively, as shown in a table 20 of FIG. 2, wherein various values of MDL—HARQ are listed according to an example of the prior art. min(x,y) returns the smaller one of x and y.
As shown in the equation Eq.1, up to min (MDL—HARQ, Mlimit) HARQ processes can be stored in the soft buffer. If the spatial multiplexing with KMIMO spatial streams is configured to the UE, each HARQ process consists of KMIMO transport blocks. Therefore, the entire soft buffer is divided into KMIMO min (MDL—HARQ,Mlimit) partitions. Each partition consists of NIR soft channel bits which can be used for storing one transport block.
Please refer to FIG. 3, which is a schematic diagram of a soft buffer SBp according to the prior art. In this example, the spatial multiplexing is not configured to the UE (i.e., KMIMO=1), the soft buffer SBp is divided into 8 partitions P301-P308 for storing 8 HARQ processes while MDL—HARQ is equal to or larger than 8. Nsoft is a size (e.g. number of bits) of the soft buffer SBp, and depends on UE category of the UE. NIR is a number of bits of a partition of the soft buffer SBp. Therefore, a transport block with a maximum size NIR can be stored in a corresponding partition, and at most 8 HARQ processes can be stored in the soft buffer SBp.
However, the UE may need to store more than 8 HARQ processes in the soft buffer in the LTE-A system when multiple CCs are configured to the UE. For example, when the UE is configured with 5 DL CCs and operates in FDD duplex mode, the UE may need to store up to 40 HARQ processes due to unsuccessful decoding of the packets. There are two possible solutions in the LTE-A system with multiple CCs, which are introduced as follows. In the first solution, the soft buffer partition rule is the same as that for the LTE system (i.e., the single CC system). In other words, up to 8 HARQ processes can be stored in the soft buffer. All the erroneous HARQ processes can share the soft buffer statistically. Thus, a blocking probability of a HARQ process increases, and the system throughput is diminished. In the second solution, the soft buffer can be simply divided into 40 partitions for storing up to 40 HARQ processes, a size of each partition of the soft buffer is reduced. For each erroneous HARQ process, the number of soft channel bits that UE can store is reduced according to the size reduction of a corresponding partition. As a result, the coding performance is reduced and more retransmissions are required, and the system throughput is diminished. Obviously, neither the first nor the second solution can achieve optimal system throughput. Therefore, when the CA is configured to the UE, how to handle a soft buffer of a UE for storing HARQ processes is a topic to be discussed and addressed.